The present invention relates to a high performance polymer material exhibiting second-order nonlinear optical properties, which can be used in ultrahigh-speed electrooptic modulators for high-capacity optical communication or frequency doubling devices for a semiconductor laser and the like, and a process for preparing the same.
A nonlinear optical material is a key material in optoelectronic technology (in a broader sense, optical technology) capable of allowing high-capacity information communication, storage and calculation at ultrahigh speed and high density which exceeds the limits of present fine technology in the field of semiconductor. Extensive research has been conducted for the development of material for semiconductor, ferroelectric inorganic crystalline and organic compound, etc. As a result, it was theoretically and experimentally verified that molecular organic compounds having conjugated .pi.-electron exhibit extremely excellent nonlinear optical properties compared with inorganic type materials as described in Chemical Review, Vol. 94, No. 1, pp. 1-278 published by the American Chemical Society in 1994. The research in this area has progressed extensively.
In particular, a side-chain type organic polymer material exhibiting second-order nonlinear optical properties, as described in U.S. Pat. Nos. 4,694,066, 4,775,574 and 4,762,912 and the like has superior processability, optical transparency, and higher nonlinear optical constant than LiNbO.sub.3 which is a commercial nonlinear optical crystal, and thereby is seen to be preferred material with which to prepare a short-wavelength optical source according to second harmonic generation and a common-waveguide type, ultrahigh-speed electrooptic modulator. For example, these materials have been used experimentally to manufacture ultrahigh-speed modulator devices having tens of GH.sub.2 bands in optical communication as described in Journal of Polymer Science, Vol. 53, pp. 649-663, 1994, etc. Such second-order nonlinear optical materials can be synthesized by direct bonding of chemical groups having high molecular hyperpolarizability to the side chain of a polymer. Thin film of these materials can be prepared by means of conventional coating technologies such as spin coating, dip coating and the like. The second-order nonlinear optical polymer material thus prepared forms an asymmetric structure of which the dipole moment of the chemical group is oriented in one direction via an electric-field poling process. Such an asymmetrically oriented structure is considerably relevant to the properties of a second-order nonlinear optical device. That is, since the properties of a device increase linearly with the orientation of the dipole forming asymmetric structure, it is very important for the preparation of a device having good performance that an electric-field poling capable of providing the device with maximum orientation and suppressing orientation relaxation during manufacture or use of the optical device. Generally, an asymmetric structure in a molecule can be formed easily by corona discharge, or by a contact poling using electrodes coated on both surfaces of a thin film. However, a certain method is required to suppress spontaneous thermal orientation relaxation of the asymmetric structure to a symmetric structure that is, in terms of energy, a stable state during manufacture or use of the optical device. Also, such asymmetric structure in a molecule with the fact that the high nonlinear optical effect appears in the region of 1.3 to 1.5 microns which is the wavelength band for optical communication, have been very considerably recognized.
Further, the significance of a very high second-order nonlinear optical constant, which is also a property of the polymers of the present invention, has been increasing since it allows the reduction of voltage applied when driving an optical device.
The method for suppressing spontaneous thermal orientation relaxation can be divided into two main classes: (1) the method of forming a crosslinked structure in a compound with heat or light when applying poling treatment and (2) the method of using a side-chain type second-order nonlinear optical polymer having a high glass transition temperature.
The method of forming a crosslinked structure in a compound using epoxy type thermosetting material with the chemical group exhibiting nonlinear optical properties is disclosed in Applied Physics Letter, Vol. 56, No. 26, pp. 2610-2613. This method provides the formation of a highly crosslinked, asymmetric oriented structure by properly controlling an electric-field poling and thermosetting reaction. Further, it was proved that the compound's nonlinear optical constant, d.sub.33 is very high (as much as 42 pm/V) and that the compound provides good suppression of the thermal orientation relaxation.
Nevertheless, a problem of the thermosetting crosslinked polymer described above is that its density distribution is uneven due to its highly crosslinked structure, and the preparation of homogeneous thin film is difficult since low molecular weight material is used in preparation thereof. Thus, light scattering is derived from polarized thermosetting thin film material. Also, up to the present, the d.sub.33 value of a highly crosslinked polymer does not reach that of inorganic materials and it is not practical to use this polymer to make an optical device.
In the method using a side-chain type, second-order nonlinear optical polymer, since high molecular material is used, a thin film which is optically homogeneous can be easily prepared by means of spin coating and the like. Also, the thin film having a high nonlinear optical constant and exhibiting superior optical properties without the loss due to light scattering can be prepared by polarizing the thin film in the vicinity of its glass transition temperature and then cooling to fix the polarized thin film. Accordingly, a side-chain type, second-order nonlinear optical polymer is considered to be a material well-suited for use in the preparation of an integrated optical device.
However, a side-chain type polymer described above is known to have the disadvantage that thermal orientation relaxation occurs when manufacturing or using a device made with the polymer. The use of a side-chain type nonlinear optical polymer having high glass transition temperature has been suggested as a solution for this problem, but other problems are encountered as the fact that electric-field poling process becomes difficult as the glass transition temperature increases and the breakage of a nonlinear optical chemical group occurs at high temperature.